Understanding spine anatomy may help patients engage more meaningfully with their diagnosis and treatment options. This glossary defines key anatomical structures, common disc conditions, and relevant biological concepts in patient-friendly language. Because each spine is different, individual presentations vary — these definitions are intended to support informed conversations with your clinical team, not replace a personalized evaluation.

Intervertebral Disc

The intervertebral disc is a cushion-like structure positioned between each vertebra in the spinal column. It functions as a shock absorber and enables spinal flexibility during daily movement. Each disc consists of two primary components: the annulus fibrosus (a tough outer ring of layered collagen fibers) and the nucleus pulposus (a gel-like inner core). Disc damage from injury or age-related degeneration is among the most common sources of chronic back pain, and symptoms can range from localized stiffness to radiating nerve pain depending on the nature and location of the damage.

Annulus Fibrosus

The annulus fibrosus is the strong outer ring of an intervertebral disc, built from multiple interlocking layers of collagen fibers arranged in a crisscross pattern. This architecture contains the nucleus pulposus and gives the disc its structural integrity under load. When tears develop in these fibrous layers — a condition known as an annular tear — inflammatory chemicals from the nucleus may leak out and irritate adjacent spinal nerve endings. Because intervertebral discs have limited blood supply, annular tears often struggle to heal without targeted intervention. To learn more, see our resource on annular tears as a root cause of back pain.

Nucleus Pulposus

The nucleus pulposus is the soft, gel-like interior of an intervertebral disc, composed primarily of water and a loose network of collagen and elastic fibers. Its high water content gives the disc its hydraulic shock-absorbing capability, allowing it to distribute pressure across the spinal column during movement. When the surrounding annulus fibrosus is compromised by a tear or degenerative thinning, the nucleus pulposus may bulge or herniate outward through the damaged layers, potentially compressing nearby nerve roots and producing pain, numbness, or weakness in the extremities.

Vertebra

A vertebra is one of the individual bones that form the spinal column, which extends from the base of the skull to the tailbone. The human spine is made up of 33 vertebrae organized into five regions: 7 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 fused sacral, and 4 fused coccygeal (tailbone) vertebrae. Each vertebra features a solid body in front and a bony arch behind, together forming the spinal canal through which the spinal cord travels. Vertebral problems such as fractures, misalignment, or bone spur formation can contribute directly to pain and neurological symptoms. For an overview of cervical vertebral anatomy, visit our cervical vertebrae anatomy guide.

Facet Joint

Facet joints are small, paired joints on the posterior surface of each vertebra that link adjacent vertebral segments. They guide spinal movement — including forward bending, extension, and rotation — while limiting excessive motion that could destabilize the spine. Each vertebra has two superior facet joints connecting to the vertebra above and two inferior facet joints connecting to the vertebra below. Like other joints in the body, facet joints are susceptible to degenerative arthritis, injury, and inflammation. When affected, they may produce localized back or neck pain that often worsens with certain positions or movements.

Foramen (Intervertebral Foramen)

The intervertebral foramen is a natural opening between adjacent vertebrae through which spinal nerve roots exit the spinal canal to travel to the rest of the body. There is one foramen on each side of the spine at every vertebral level. When a disc herniates or bulges, or when bone spurs or thickened ligaments develop nearby, these openings can narrow — a condition called foraminal stenosis. Narrowing may compress the exiting nerve root, producing pain, numbness, tingling, or weakness in the arms, legs, or other areas served by that nerve.

Annular Tear

An annular tear is a fissure or rip in the annulus fibrosus — the layered outer ring of an intervertebral disc. These tears vary in depth, from superficial surface cracks to deep lacerations that penetrate multiple layers. Because the outer annulus contains pain-sensitive nerve endings, even contained tears can generate significant discogenic pain. Additionally, tears allow pro-inflammatory chemicals from the nucleus pulposus to contact these nerve endings, amplifying discomfort. Due to the disc’s limited vascular supply, annular tears frequently do not heal on their own and may progress to further degeneration over time. Our article on whether annular tears cause chronic low back pain explores this further.

Degenerative Disc Disease (DDD)

Degenerative Disc Disease describes the progressive breakdown of one or more intervertebral discs over time. Despite its clinical name, it is more accurately characterized as a wear-and-tear process than a traditional disease. DDD typically involves loss of disc height, dehydration of the nucleus pulposus, and the development of annular tears or fissures, all of which reduce the disc’s capacity for shock absorption and spinal flexibility. While some degree of disc degeneration is a natural part of aging, in many patients it may lead to chronic back pain, stiffness, and heightened risk of disc herniation — particularly when degeneration is advanced or causes segmental instability. For treatment perspectives, see when conservative care stops working for DDD.

Herniated Disc

A herniated disc occurs when the nucleus pulposus — the gel-like core of the disc — pushes through a rupture or tear in the annulus fibrosus and extends beyond the disc’s normal boundary. This is also commonly called a “ruptured disc” or “slipped disc.” Herniated disc material can directly compress nearby spinal nerve roots or, in severe cases, the spinal cord itself, producing sharp radiating pain, numbness, tingling, or muscle weakness in the area served by the affected nerve. Herniations may develop suddenly from acute injury or gradually through cumulative degenerative changes. Our resource on bulging disc vs. herniated disc differences clarifies how these two presentations differ.

Bulging Disc

A bulging disc occurs when an intervertebral disc extends beyond its normal perimeter without the nucleus pulposus rupturing through the annulus fibrosus. Unlike a herniation, the inner gel remains contained while the disc’s outer wall protrudes outward in a more generalized fashion. Although the disc has not ruptured, the bulge may still press against adjacent nerve roots, the spinal cord, or other structures within the confined space of the spinal canal. Symptoms can range from localized back pain to radiating discomfort, numbness, or tingling, depending on which neural structures are involved.

Sciatica

Sciatica is pain that radiates along the pathway of the sciatic nerve — the largest nerve in the body — typically originating in the lower back and traveling through the buttock and down the back of one leg. It is caused by compression or irritation of one or more of the lumbar nerve roots that form the sciatic nerve, often due to a herniated disc, bone spur, or spinal stenosis. Many patients describe sciatic pain as shooting, burning, or electric in character, sometimes accompanied by numbness, tingling, or muscle weakness anywhere along the nerve’s course. For a patient-focused overview, see our article 10 myths about sciatica and non-surgical relief.

Radiculopathy

Radiculopathy describes a cluster of symptoms arising from compression, irritation, or inflammation of a spinal nerve root as it exits the vertebral column. Common causes include herniated discs, spinal stenosis, bone spurs, and other degenerative changes. Symptoms typically follow the distribution of the affected nerve root: cervical radiculopathy may produce pain, numbness, tingling, or weakness in the shoulder, arm, or hand, while lumbar radiculopathy often manifests as similar symptoms in the buttock, leg, or foot. Understanding radiculopathy helps clarify why a spinal problem can generate symptoms felt far from the spine itself.

Discogenic Pain

Discogenic pain refers to pain that originates directly from a damaged or degenerated intervertebral disc — distinct from pain caused purely by nerve compression. It often presents as a deep, aching discomfort in the back, sometimes referring to the buttocks or thighs. Activities that load the spine axially — such as prolonged sitting, bending forward, lifting, or coughing — tend to aggravate discogenic pain. Internally, it frequently arises from annular tears that allow inflammatory mediators to irritate the pain-sensitive nerve fibers within the disc’s outer layers. Identifying discogenic pain as the primary driver is important because it may respond better to treatments aimed at addressing the disc directly rather than managing symptoms alone.

Spinal Stenosis

Spinal stenosis is a narrowing of the spinal canal or the intervertebral foramina, which can compress the spinal cord or exiting nerve roots. It most commonly results from age-related degenerative changes including bulging discs, osteophyte (bone spur) formation, or thickening of the spinal ligaments. Compression of neural structures may produce pain, numbness, tingling, or weakness in the back, buttocks, or legs. A characteristic pattern seen in many patients is symptom worsening with standing or walking and partial relief with sitting or forward flexion. For an in-depth review, see 10 common symptoms of spinal stenosis and our overview of non-surgical treatments for spinal stenosis.

Fibrin

Fibrin is a naturally occurring insoluble protein central to the body’s wound-healing and clotting processes. It forms a mesh-like scaffold that stops bleeding and provides structural support for new tissue growth and repair. In regenerative disc care, fibrin’s biological properties may be harnessed therapeutically. During an intra-annular fibrin injection, fibrin is delivered into the damaged disc to help seal annular tears and create a scaffold that may support the body’s own tissue-repair mechanisms. This non-surgical approach aims to address the structural source of discogenic pain rather than simply masking symptoms. To explore how fibrin-based treatment works in practice, see our overview of annular tear repair as a non-surgical approach.

Expert Take

Familiarity with spine anatomy and terminology can help patients ask more targeted questions during consultations and better understand imaging reports or treatment recommendations. In our clinical team’s experience, patients who understand the distinction between discogenic pain and nerve-compression pain — and who recognize terms like annular tear, nucleus pulposus, or foraminal stenosis — tend to engage more actively in shared decision-making. That said, every individual’s anatomy and pain presentation is different, and self-education should complement, not replace, a thorough personalized evaluation.